1. Field of the Invention
This invention relates to a phosphor which exhibits a visible emission under excitation of infrared radiation.
2. Description of the Prior Art
It has recently been found that some phosphors comprising as a host material a rare earth fluoride such a yttrium fluoride, lanthanum fluoride, and lutetium fluoride as an activator, a rare earth ion such as erbium (Er.sup.3+) for forming the luminescence center and as a sensitizer an ytteribium ion (Yb.sup.3+), exhibit a line emission throughout the visible region such as green, red or blue emission due to the trivalent rare earth ion acting as the activator under excitation of infrared radiation from GaAs infrared emitting diodes and the like.
In addition to the above-mentioned yttrium fluoride, lanthanum fluoride and lutetium fluoride, barium yttrium fluoride is known as the host material effective as an element for infrared-to-visible conversion. Phosphors having these fluorides as the host material exhibit considerable luminescence efficiency under excitation of infrared radiation, but because of insufficiency in luminescence brightness, their positive application to various solid state light emitting display devices such as a character display or multicolor display device has been hindered.
Particularly instructive references are included in the following patent and publication:
A. U.S. Pat. No. 3,541,018 patented on Nov. 17, 1970 R. A. Hewes. This patent discloses an infrared-excitable ytterbium-sensitized erbium- or thulium-activated rare earth fluoride luminescent material. Although the phosphors of this invention are similar to the luminescent material of the patent with respect to the intended functions, but, the electronic process in solid on the excitation-to-emission mechanism is substantially improved by changing the compositions over the material of said patent, whereby the luminescence efficiency is extremely heightened. PA1 b. Mat. Res. Bull, vol. 4, page 777 (1969), L. G. Van Uitert et al.
This publication teaches that an addition of beryllium fluoride is effective to some extend in the preparation of phosphors. However, because of deliquescence and toxicity of such additive it is impractical for the preparation of phosphors. The product of this invention has no such disadvantage at all, but gives a higher luminescence efficiency.